I try few hours to find why my search don’t work correctly. I use “page navi” plugin but the error persist without plugin too.
Finally, after many code modification I fout the problem. It was in the POST method who must be replaced with GET method.
But attention, Dilectio wordpress themes have two forms in their code. One is simple to find, is in searchform.php but the search field placed in the sidebar on the blog is in sidebar.php
So you must have this code on form:
<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="get">